1. Field of the Invention
The present invention relates to a short-circuit protective circuit for protecting a power Darlington transistor in case where a load is short-circuited, and to a power Darlington transistor module containing the short-circuit protective circuit therein.
2. Prior Art
In a power circuit device like a power Darlington transistor, operating conditions on the power circuit device when a load therein is short-circuited, that is, voltage and current, are ones of the most significant factors which affect a determination on whether the power circuit device can be used. Especially, when a load is short-circuited in a power transistor utilized for an inverter, current 4 to 10 times as much as rated current flows in the transistor under the condition that supply voltage is applied thereto. Power applied to the transistor is approximately 1000 times as large as rated power demand. Thus, if this short-circuiting state is retained, the transistor is at last broken. This phenomenon is named "load short-circuit breakdown", which is generally one of the most important factors whether a transistor can be adapted or not.
In practice, the power transistor is required to sustain a state where a load is short-circuiting (load short-circuiting state) lasting for at least 10.about.20 .mu.sec or more. For this period of time a drive circuit for the transistor can detect and control an abnormal increase in current flowing in the load so as to break a transistor circuit, and hence, the breakdown of the transistor can be avoided.
FIG. 8 depicts a circuit (a test circuit) for a test about the "load short-circuit breakdown". In this test circuit, a power source 2 and a capacitor 3 are connected in parallel to a collector C and emitter E of a transistor 1 or a measured object. It is important that the wiring connecting the transistor 1, the power source 2 and the capacitor 3 to one another must be as short as possible. Upon the test, supply voltage V.sub.cc is set to a certain level, and base current I.sub.B of the transistor 1 is supplied in a single pulse having a width of 30 .mu.sec to turn on the transistor 1 for an instant. Then, the maximum value of collector current I.sub.C and collector-emitter voltage V.sub.CE at that instant are measured.
FIG. 9 is a graph showing results of a test where the test circuit shown in FIG. 8 is used to conduct a measurement to a measured sample of the prior art transistor 1 without a protective circuit. For three sorts of the base current I.sub.B, respective maximum values of the collector current I.sub.C are measured while the supply voltage V.sub.cc is raised stepwise till the transistor 1 is broken down. The vertical axis of FIG. 9 represents the maximum value of the collector current I.sub.C in such a condition. Symbol X in FIG. 9 represents a point where the transistor 1 is broken down. As can be seen in FIG. 9, it is apparent that as the base current I.sub.B becomes smaller, the collector current I.sub.C becomes smaller related to the same collector-emitter voltage V.sub.CE, and consequently, the level of the collector-emitter voltage V.sub.CE where the transistor 1 is broken down (breakdown voltage) becomes higher.
The "load short-circuit breakdown" of the transistor is classified into two sorts; a first breakdown mode (thermal breakdown mode) caused by temperature rising in a semiconductor chip containing transistors, and a second breakdown mode (power breakdown mode) in which the transistor is broken down at the instant when electric power applied to the transistor reaches a specified level (see Document 1: H. Nishiumi et al., "High Voltage High Power Transistor Modules for 440 V AC Line Voltage Inverter Applications", in Conference Record of IPEC-Tokyo 83, pp. 297-305, 1983). With the test circuit shown in FIG. 8, since power is applied to the transistor 1 for a very short period of about 30 .mu.sec, the breakdown point shown in FIG. 9 is equivalent to the power breakdown mode.
A method of improving sustainability against breakdown in the power breakdown mode (breakdown sustainability) under the condition of "load short-circuiting state" is, as will be recognized in the above results of the test, basically restricting the collector current I.sub.C so as not to cause a excessively large flow of the collector current I.sub.C, and for that purpose, the base current I.sub.B may be reduced in load short-circuiting. An exemplary method for that utilizes a general characteristic of the transistor that base-emitter forward voltage V.sub.BE rises as the collector current I.sub.C is increased. Specifically, a reference voltage circuit preset appropriately is provided to make a comparison between reference voltage supplied by this circuit and the base-emitter forward voltage V.sub.BE of the transistor. If the base emitter forward voltage V.sub.BE is higher, the base current I.sub.B of the transistor is reduced to properly control the excessive flow of the collector current I.sub.C. FIGS. 10 to 12 depict three types of embodiments based upon this method.
&lt;First Prior Art Embodiment&gt;
As to an exemplary circuit shown in FIG. 10, a short-circuit protective circuit 6 comprised of a transistor 5 and voltage dividing resistances R1 and R2 for retaining voltage at a specified level or below is provided between a base (initial stage base) B and emitter (final stage emitter) E of a Darlington transistor 4. A free wheel diode 7 is connected in parallel with the Darlington transistor between its collector C and emitter E. In a situation as in the "load short-circuit" where collector current I.sub.C (primary current) several times as much as the rated current flows, however, base-emitter forward voltage of the Darlington transistor 4 rises. When voltage obtained by dividing the base-emitter forward voltage by the voltage dividing resistances R1 and R2 reaches base-emitter forward voltage of the transistor 5, base current I.sub.B of the Darlington transistor 4 is bypassed to the transistor 5, and the base current I.sub.B supplied to the Darlington transistor 4 is suppressed. Consequently, excessive rising of the collector current I.sub.C of the Darlington transistor 4 can be suppressed.
&lt;Second Prior Art Embodiment&gt;
In a prior art embodiment shown in FIG. 11, the voltage dividing resistances R1 and R2 in FIG. 10 are replaced with a diode 8 connected between a base of the transistor 5 and a base B of the Darlington transistor 4. The short-circuit protective circuit 6 functions so that the base-emitter forward voltage of the Darlington transistor 4 is not more than the total of the sum of forward threshold voltage of the diode 8 and the base-emitter forward voltage of the transistor 5. Instead of the diode 8 connected in a forward direction, a Zener diode connected in the reverse direction may be used for the similar feature. The diode 8 may be replaced with a circuit having a forward diode and a reverse Zener diode connected in series to obtain the similar feature.
&lt;Third Prior Art Embodiment&gt;
In a prior art embodiment shown in FIG. 12, a resistance 10 is connected between an emitter E of the Darlington transistor 4 and an output terminal EE close to the emitter, and a diode 9 is connected in parallel with the Darlington transistor 4 between a base B of the Darlington transistor 4 and the output terminal EE. As collector current I.sub.C of the Darlington transistor 4 increases and its base-emitter forward voltage becomes higher than the sum of forward threshold voltage of the diode 9, base current I.sub.B of the Darlington transistor 4 is bypassed to the diode 9. Although it is generally difficult to adequately regulate the forward threshold voltage of the diode 9, voltages at opposite terminals of the resistance 10 increase in proportion to an increase in the collector current I.sub.C of the Darlington transistor 4, and hence, in this prior art embodiment, there can be a wide choice of levels of the forward threshold voltage of the diode 9. Also in this prior art embodiment, the diode 9 connected in a forward direction may be replaced with a Zener diode connected in the reverse direction, and additionally, it may be replaced with a serial circuit of a forward diode and a reverse Zener diode.
However, the above prior art technology has disadvantages as mentioned below.
&lt;Disadvantage in the First Prior Art Embodiment&gt;
In the first prior art embodiment, it is necessary to set levels of the voltage dividing resistances R1 and R2 considerably low so that base current corresponding to a current amplification factor hFE of the transistor 5 can be sufficiently supplied to the transistor 5. When the levels of the voltage dividing resistances R1 and R2 are low, the base current I.sub.B must be raised in the circuit shown in FIG. 10, and a great amount of power is required to drive the circuit. Thus, there arises the problem that a driving device (omitted in the drawing) for driving the circuit is costly.
When the prior art exemplary circuit is applied to an inverter, for example, feedback current flows in a free wheel diode 7 for a while. For this period reverse bias voltage is applied between the base B and emitter E of the Darlington transistor 4 to turn off the Darlington transistor 4, and this allows the transistor 5 to work as a reversely connected transistor; that is, base current of the transistor 5 is supplied through the voltage dividing resistance R2, and current flows in a course from an emitter of the transistor 5 via a collector of the transistor 5 and the base B of the Darlington transistor 4 to the collector C of the Darlington transistor 4. Although the current is minuter than the primary current I.sub.C, there is a problem that the current easily causes the breakdown of the Darlington transistor 4 in an inverter which performs switching at high speed. This is because the Darlington transistor 4 is put in a situation where supply voltage is quickly applied thereto just after the period when the feedback current flows in the free wheel diode 7, and even with a slight electric charge remaining in a device in this situation, current equivalent to the multiple of the electric charge flows in the Darlington transistor 4 (see Appended Document 1).
&lt;Disadvantage of the Second Prior Art Embodiment&gt;
The second prior art embodiment has a disadvantage that it is difficult to select an appropriate level of the sum of the forward threshold voltage of the diode 8. Moreover, selection of the level is difficult the more because it must be performed corresponding to a temperature range from the ambient temperature to 125.degree. C., or a temperature range in which a device is operative. There also arises a problem that the diode 8 must be selected for each of Darlington transistors of various numbers of stages. Furthermore, there is another disadvantage that oscillation is easily caused because of a high impedance of the base of the transistor 5. To avoid this, a resistance is generally connected between the base and emitter of the transistor 5. However, this causes residual electric charge as in the first prior embodiment, and thus, there is another disadvantage that this prior art exemplary circuit cannot be applied to an inverter.
&lt;Disadvantage of the Third Prior Art Embodiment&gt;
In the third prior art embodiment, there is a disadvantage that it is not easy to set the forward threshold voltage of the diode 9 to an appropriate level in the above-mentioned working temperature range although its adjustment is relatively easy because a resistance 10 is provided. There also arises a disadvantage that the resistance 10 causes an extra power loss because the primary current I.sub.C flows in the resistance 10.